The present invention relates broadly to a filter apparatus and in particular to a multi-mode filter apparatus having mode selectivity.
Microwave transmission lines are used for transmission of electromagnetic energy at microwave frequencies from one point to another. A transmission line may be defined as a system of material boundaries which forms a continuous path from one place to another and is capable of directing the transmission of electromagnetic energy along this path. At microwave frequencies, a wide variety of metallic and dielectric structures are used, the choice depending upon the specific application and frequency range. The wavelengths at microwave frequencies are small, ranging from a few millimeters to under 1 m. It is therefore typical of microwave transmission lines that even when they are physically rather short, their length measured in electrical wavelengths ranges from an appreciable fraction of a wavelength to many wavelengths. For this reason, if microwave transmission lines are not carefully designed, substantial losses of energy by radiation, reflection, and attenuation may be encountered.
Microwave filters are operated at microwave frequencies which are generally considered to be above 1 GHz. However, they may also be used in the much lower high-frequency (hf) range of 3-30 MHz. The components are no longer lumped elements, but consist of sections of transmission lines, strip lines, coupled lines, and waveguide structures. A form of microwave filter is the overmoded circular cylindrical resonant cavity diode combiner which may provide a possible alternative to microwave power tubes. As the required power output increases, the number of diodes and the cavity dimensions increase. As the dimensions increase, the number of higher order modes increases. The higher order modes should be suppressed to keep the frequency spectrum clean and the power in the lowest possible mode consistent with the required frequency.